The specific aims of this application are: (1) to characterize the various species of human Ep isolated from different anemic patients, (2) to raise a library of monoclonal antibodies against these various species, (3) to define the functional and antigenic domains of Ep molecules and (4) to identify isolate, and characterize Ep receptor(s). Our long term goals are: (1) to define the mechanisms of action of Ep, elucidating the specific interaction of Ep with its target cells, and the subsequent responses of these cells, and (2) to gain an understanding of the molecular basis of various anemias, and eventually contribute to their diagnosis and treatments. The following methods would be used to achieve our specific aims: hybridoma techniques for the generation of a library of monoclonal antibodies, immunochemical mapping for defining the functional and antigenic domains of Ep, and photoaffinity cross-linking for receptor studies.